This invention relates to capacitive accelerometers and, in particular, to microelectromechanical capacitive accelerometers and methods of making same.
Accelerometers are required in numerous applications, such as navigation, guidance, microgravity measurements, seismology and platform stabilization. Also, as they become manufacturable at low cost with small size, they attain a large potential consumer market in their application as a GPS-aid to obtain position information when the GPS receivers lose their line-of-sight with the satellites.
Some accelerometers are fabricated by surface micromachining or bulk micromachining. The surface micromachined devices are fabricated on a single silicon wafer. However, they generally have low sensitivity and large noise floor, and thus cannot satisfy requirements of many precision applications.
Some high resolution accelerometers are bulk micromachined and use multiple wafer bonding as part of their manufacturing process. This wafer bonding is a complex fabrication step, and hence results in lower yield and higher cost. Also, forming damping holes in the thick bonded wafers is difficult, and thus special packaging at a specified ambient pressure is typically needed to control the device damping factor. Finally, due to wafer bonding, these devices show higher temperature sensitivity and larger drift.
U.S. Pat. No. 5,345,824 discusses a monolithic capacitive accelerometer with its signal conditioning circuit fabricated using polysilicon proofmass and surface micromachining.
U.S. Pat. No. 5,404,749 discusses a boron-doped silicon accelerometer sensing element suspended between two conductive layers deposited on two supporting dielectric layers.
U.S. Pat. No. 5,445,006 discusses a self-testable microaccelerometer with a capacitive element for applying a test signal and piezoresistive sense elements.
U.S. Pat. No. 5,461,917 discusses a silicon accelerometer made of three silicon plates.
U.S. Pat. No. 5,503,285 discusses a method for forming an electrostatically force rebalanced capacitive silicon accelerometer. The method uses oxygen implantation of the proofmass to form a buried oxide layer and bonding of two complementary proofmass layers together. The implanted oxide layer is removed after bonding to form an air gap and release the proofmass.
U.S. Pat. No. 5,535,626 discusses a capacitive microsensor formed of three silicon layers bonded together. There is glass layer used between each two bonded silicon pairs.
U.S. Pat. No. 5,540,095 discusses a monolithic capacitive accelerometer integrated with its signal conditioning circuitry. The sensor comprises two differential sense capacitors.
U.S. Pat. No. 5,559,290 discusses a capacitive accelerometer formed of three silicon plates, attached together using a thermal oxide interface.
U.S. Pat. No. 5,563,343 discusses a lateral accelerometer fabricated of a single crystal silicon wafer.
U.S. Pat. No. 5,605,598 discloses a monolithic micromechanical vibrating beam accelerometer having a trimmable resonant frequency and method of making same.
The paper entitled xe2x80x9cAdvanced Micromachined Condenser Hydrophonexe2x80x9d by J. Bernstein et al, Solid-State Sensor and Actuator Workshop, Hilton Head, S.C., June, 1994, discloses a small micromechanical hydrophone having capacitor detection. The hydrophone includes a fluid-filled variable capacitor fabricated on a monolithic silicon chip.
The paper entitled xe2x80x9cHigh Density Vertical Comb Array Microactuators Fabricated Using a Novel Bulk/Poly-Silicon Trench Refill Technologyxe2x80x9d, by A. Selvakumar et al., Hilton Head, S.C., June 1994, discloses a fabrication technology which combines bulk and surface micromachining techniques. Trenches are etched and then completely refilled.
Numerous U.S. patents disclose electroplated microsensors such as U.S. Pat. Nos. 5,216,490; 5,595,940; 5,573,679; and 4,598,585.
Numerous U.S. patents disclose accelerometers such as U.S. Pat. Nos. 4,483,194 and 4,922,756.
U.S. Pat. No. 5,146,435 discloses an acoustic transducer including a perforated plate, a movable capacitor plate and a spring mechanism, all of which form a uniform monolithic structure from a silicon wafer.
An object of the present invention is to provide a microelectromechanical capacitive accelerometer wherein at least one of its conductive electrodes includes a layer which is stiff but thin relative to a proofmass of the accelerometer.
Another object of the present invention is to provide a microelectromechanical capacitive accelerometer formed from a single semiconductor wafer with a proofmass having a thickness substantially equal to the thickness of the wafer, controllable/small damping and large capacitance variation.
Yet another object of the present invention is to provide a microelectromechanical capacitive accelerometer having at least one conductive electrode formed as a stiffened film or layer which is thin by at least one order of magnitude relative to the thickness of a proofmass of the accelerometer.
Yet still another object of the present invention is to provide a low cost method for making a microelectromechanical capacitive accelerometer wherein at least one resulting conductive electrode is relatively thin but stiff and the resulting proofmass is relatively thick so as to provide high sensitivity in the capacitive accelerometer.
In carrying out the above objects and other objects of the present invention, a microelectromechanical capacitive accelerometer having an input axis is provided. The accelerometer includes at least one conductive electrode including a planar layer which is relatively thin along the input axis. The at least one conductive electrode is stiff so as to resist bending movement along the input axis. The accelerometer also includes a proofmass which is thicker than the planar layer by at least one order of magnitude along the input axis and a support structure for supporting the proofmass in spaced relationship from the at least one conductive electrode. The at least one conductive electrode and the proofmass have a substantially uniform narrow air gap therebetween. The conductive electrode and the proofmass form an acceleration-sensitive capacitor.
Preferably, the at least one electrode is sufficiently stiff to force-balance proof-mass displacement due to acceleration along the input axis without substantial bending of the at least one conductive electrode along the input axis.
Also, preferably, the proofmass is formed from a single silicon wafer having a predetermined thickness and wherein the thickness of the proofmass is substantially equal to the predetermined thickness.
In one embodiment, the planar layer is dimensioned and is formed of a material so that the at least one conductive electrode is stiff along the input axis. The planar layer may be a metallized planar layer.
The at least one conductive electrode may include a plurality of stiffeners extending from the planar layer along the input axis to stiffen the at least one conductive electrode. In a preferred embodiment, the stiffeners extend either away from or towards the proofmass from the planar layer. The proofmass includes a plurality of cavities when the stiffeners extend toward the proofmass. In this embodiment, the stiffeners are received within the cavities and the stiffeners and the proofmass have the substantially uniform narrow air gap therebetween.
The planar layer and the stiffeners may be formed of different materials or different forms of the same material such as a silicon semiconductor material.
The planar layer and the proofmass may be formed of different materials or different forms of the same material.
Further in carrying out the above objects and other objects of the present invention, a microelectromechanical capacitive accelerometer having an input axis is provided. The accelerometer includes a pair of spaced conductive electrodes. Each of the conductive electrodes includes a planar layer which is relatively thin along the input axis but is stiff to resist bending movement along the input axis. The accelerometer also includes a proofmass which is thicker than each of the planar layers by at least one order of magnitude along the input axis and a support structure for supporting the proofmass between the conductive electrodes. The conductive electrodes and the proofmass form a pair of substantially uniform narrow air gaps on opposite sides of the proofmass. The pair of conductive electrodes and the proofmass form a pair of acceleration-sensitive capacitors.
Preferably, both of the conductive electrodes are sufficiently stiff to force-balance proof-mass displacement due to acceleration along the input axis without substantial bending of the conductive electrodes along the input axis.
Also, preferably, the support structure includes a plurality of conductive beams for suspending the proofmass between the conductive electrodes.
Still, preferably, each of the planar layers of the conductive electrodes has a plurality of damping holes formed completely therethrough to reduce damping factor.
In a preferred embodiment of the invention, the proofmass is formed from a single silicon wafer having a predetermined thickness and wherein the thickness of the proofmass is substantially equal to the predetermined thickness.
In one embodiment, the planar layer is dimensioned and is formed of a material so that the at least one conductive electrode is stiff along the input axis.
Each of the planar layers may be a metallized planar layer.
Each conductive electrode may include a plurality of stiffeners extending from its planar layer along the input axis to stiffen their respective conductive electrode. The stiffeners may extend either towards or away from the proofmass from their respective planar layer. When the stiffeners extend toward the proofmass, the proofmass includes a plurality of cavities on opposite sides thereof and the stiffeners are received within the cavities. In this embodiment, the stiffeners and the proofmass have the substantially uniform narrow air gaps therebetween.
The planar layers and the stiffeners may be formed of different materials or different forms of the same material such as a silicon semiconductor material.
The planar layers and the proofmass may be formed of different materials or different forms of the same material.
Yet still further in carrying out the above objects and other objects of the present invention, in a method for making a microelectromechanical capacitive accelerometer having a proofmass with a thickness along an input axis of the accelerometer and at least one conductive electrode from a single semiconductor wafer having a predetermined thickness, an improvement is provided. The improvement includes the steps of depositing a planar layer which is relatively thin along the input axis on the wafer and stiffening the planar layer to form the at least one conductive electrode which is stiff so as to resist bending movement along the input axis. The method also includes the step of forming a substantially uniform narrow gap between the at least one conductive electrode and the proofmass wherein the thickness of the proofmass is at least one order of magnitude greater than the thickness of the planar layer.
Preferably, the thickness of the proofmass is substantially equal to the predetermined thickness.
In one embodiment, the semiconductor wafer is a silicon wafer.
The accelerometer of the present invention preferably utilizes stiffened thin film sense/feedback electrodes and the method of the present invention is provided to overcome the challenges of high precision capacitive accelerometer design. These challenges are achieving a large proofmass, controllable and low damping factor, and large capacitance variationxe2x80x94all by the same design. The accelerometer has the following features: high resolution, large sensitivity, small damping without vacuum packaging, and low cross-axis sensitivity. Also, it has low temperature sensitivity and good long term stability as it is all silicon in one embodiment and made in a single wafer process. The manufacturing method of the invention provides batch fabrication at low cost and with high yield.
Several significant innovative features of the accelerometer structure and its manufacturing technique are: 1) forming the top and bottom sense/feedback electrodes with embedded damping holes using stiffened deposited polysilicon layers; 2) forming a thick proofmass by etching bulk of a single silicon wafer; 3) forming a uniform narrow gap over a large area by etching a sacrificial layer; and 4) using a single silicon wafer for manufacturing without any need for wafer bonding.
The microaccelerometer preferably includes a silicon-wafer thick proofmass suspended with conductive beams between two conductive electrodes on top and bottom. The electrodes and the proofmass are separated by a narrow air gap and form two acceleration sensitive capacitors on top and bottom. The electrodes are made stiff to be able to force rebalance the proofmass displacement due to external acceleration and operate the sensor in closed-loop. These electrodes have embedded damping holes to control the damping factor and effectively reduce the device noise floor at atmospheric ambient pressurexe2x80x94without any need for vacuum packaging.
In one embodiment, the electrodes are formed by thin polysilicon film deposition. There are vertical stiffeners embedded in the electrodes to make them stiff in the sense direction. The vertical stiffeners are made by complete refilling of the vertical trenches etched in the proofmass. A thin sacrificial silicon oxide layer separates the proofmass from the electrodes and will be etched away after proofmass formation in silicon etchant. The conductive suspension beams are formed by shallow p++ diffusion or polysilicon. The p++ beams are not etched in the silicon etchant during the proofmass formation and release. The polysilicon beams are protected by a thin silicon oxide layer, similar to the polysilicon electrodes.
The sensor typically is operated in a closed-loop mode. Preferably, a switched-capacitor sigma-delta modulator circuit is utilized to force-rebalance the proofmass and provide direct digital output for the accelerometer.
The above objects and other objects, features, and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings.